National Grid began a lockout of its unionized gas workers in Massachusetts on Monday after months of failed contract negotiations.

About 1,100 gas workers employed by National Grid in Massachusetts have been locked out of work beginning Monday after months of attempted contract negotiations. The international power company and its unionized gas workers failed to reach an agreement on issues regarding employee health care, pension cuts and retirement, overtime compensation and outsourcing work to contractors.

Steel unions Local 12003 and Local 12012 said National Grid failed to negotiate "a fair contract that recognizes the crucial services these workers provide across Massachusetts." The locked out gas workers cover more than 85 cities and towns across Massachusetts.

"National Grid continues to push proposals that threaten public safety and drive down wages. The proposed cuts come as National Grid seeks tens of millions of dollars from Massachusetts consumers in its upcoming rate case, and as the company received a major tax cut from the Trump Administration," union spokespeople said in a statement.

The unions argues that any replacement of experienced gas workers with outside contractors, whether temporarily for a lockout or long-term, poses a safety threat to the public due to the dangerous nature of working with live gas lines.

National Grid said in a statement that "safe and reliable natural gas service will continue" despite the lockout.

According to National Grid spokespeople, existing agreements that carried on during months-long negotiations expired at midnight on June 24. The company said both parties negotiated five hours past that deadline, to no avail.

Union representatives say National Grid rejected workers' request to continue working while bargaining continues. A National Grid spokesperson said because there is no contract in place, the company will not accept the risk of employees working and walking off the job at any point.

"We have been bargaining in good faith for months, and we believe the benefits in our last proposal were robust," Marcy Reed, president of National Grid in Massachusetts, said in a statement.

According to union spokesperson Michael Sherry, part of the failed contract negotiations included pension cuts for new hires.

"[Pension cuts] are not only wrong for new hires, but inevitably a gateway to pension cuts and limitations for existing members as well," Sherry told MassLive.

The company is enacting a "comprehensive work contingency plan" in Massachusetts during the lockout, which includes the deployment of its management employees and outside contractors to staff natural gas operations.

Unionized gas workers plan to congregate in Malden Monday morning on the first day of the lockout. In 2016, a similar dispute occurred when Mass. gas workers rejected National Grid contract offer that would replace pensions with 401(k) plans for new hires.